A "Cross-section" and "half-life" of excited states

[DenisH]

TL;DR Summary

I am searching to anticipate ionization yield in 2+1 photons resonant ionization as a function of the excited state. <moved>

Hello, I work with a spectrometer that does ionizations through laser 2+1 photons resonant ionization (a high power narrow bandwidth laser is tuned to a precise wavelenght so that it allows reaching an excited energy level of a particular element with the sum of two photons absorbed concomitantly by the atom, then a third photon allows ionization).

I am trying to anticipate with excited state would lead to the better yields of ionization. I imagine ionization efficiency will depend on the cross-section of absorption of the photons by the element and by the life duration of the excited state (as far as it as a life duration higher than the time delay before statistically getting hit by a third photon).

My question thus are : can the cross-section and/or "half-lives" of energy levels be determined from the nature of the electronic transition and its energy? At least comparatively as a function of the state? Will the cross-section be energy or level dependant?

 

[Twigg]

I assume you have some means of measuring the resulting number of ions produced.

To measure the cross-section, you additionally need to know your neutral atom number and the incident intensity for the ionization photon.

You can measure the lifetime of the excited state if you have a fast way of turning on/off the light. Turn on the 2 photons, wait for a fixed time, then turn on the 3rd photon and count how many ions were produced. The number of ions produced should decay as the excited state population decays. The hard part is being able to turn the light on/off on a timescale that is fast compared to the lifetime (ns, probably). If you are working with pulsed lasers, you are already all set! But since you say narrow line and high power, I have a feeling you are working with CW lasers.

 

[DenisH]

Thank you for your answer Twigg.

The approach you propose Twigg would work if the difference between the possible excited states and the ionization barrier energy would be higher than that of the UV photons... this is not our case and the third ionization photon comes from the same UV pulse.
Anyway, this demonstrates that I should had given more context and details about my problem: I indeed have a Resonant Ionization - Time of Flight spectrometer initially developed for krypton ultratraces quantification, and I envisage to modify it for xenon analysis. For krypton an excited state is reached with two photons of ~216.6 nm. Around this wavelength, many excited states of Xe are expected, including one that can be reached at ~216.8 nm... very close to our Kr state. We indeed have a resonant ionization of Xe detected by mass spectrometry with the ~216.8 nm photons but the sensitivity is somewhat 1 order of magnitude below that of Kr. Tuning of the UV 10ns pulsed ionization laser wavelength is done by a dye laser... shifting by 0.2nm its wavelength is quite easily done experimentally without losing too much efficiency, but if we want to explore the whole ~215 to ~220 nm the dye laser is supposed to cover, advanced adjustments of the dye laser (requiring a costful intervention from the supplier) would have to be done.
Therefore, I would prefer to anticipate which excited states would be the best candidates before engaging money and time in this endeavour. A first obvious filter is to select even parity. Then I can imagine life expectancy of the excited state can be problematic if the average time before a third photon in the laser pulse hit the atom is higher than the state half-life. But more probably as I feel it, what would reduce ionization efficiency is the "cross-section" of the state (direct probability of absorbing the photons but also the energy width of the state)... and here is where I get lost. Thus again (but I hope better said this time): given the nature of an excited state, can we anticipate its ionization efficiency? I looked into the literature for that and actual values for xenon but either there is nothing or more likely I lack the expertise, vocabulary and expertise of the field to find and comprehend the relevant infos.
Thanks again

 

[Twigg]

The approach you propose Twigg would work if the difference between the possible excited states and the ionization barrier energy would be higher than that of the UV photons... this is not our case and the third ionization photon comes from the same UV pulse.

I don't understand how the ionization threshold matters. What I was suggesting was to vary the delay between the 2-photon UV pulse and the ionization UV pulse. As the delay gets longer, more atoms will decay out of the excited state and so you'll see fewer ions on your time-of-flight readout. Am I missing something?

and the third ionization photon comes from the same UV pulse.

If the two pulses aren't spatially overlapped, you can introduce a variable delay line between them (two mirrors, one of them sliding, with several meters of optical path accumulated over several passes). If they are spatially overlapped, yeah that's tough.

Although I see in hindsight that you're trying to predict without needing time-consuming measurements and expensive laser surgeries. My bad.

I feel that states that have $\Delta J = +2$ above the ground state will have fewer dipole-allowed decay pathways than states with $\Delta J = 1$ or $0$ above the ground state. Just a thought.